Southern Permian Basin Research Articles

Controls on Clay Mineral Distribution and Chemistry in the Early Permian Rotliegend of Germany

AbstractAuthigenic clay minerals in the German Rotliegend formed mainly at burial depths >1.5 km. Illite is the dominant cement, although kaolinite, dickite and early radial chlorite are abundant locally. Illites contain more AI and late chlorites more AI and Fe in sequences showing extensive grain dissolution. This relationship between clay chemistry and grain dissolution suggests that clay cementation is linked to grain dissolution. Sequences at relatively shallow burial depths (<3-5 km) contain less clay cement. In the more deeply buried sections, increased illite and kandite cementation, together with extensive grain dissolution, is evident where the Rotliegend is juxtaposed against Carboniferous Coal Measures. Faults also acted as important conduits for acidic fluids. Illite and kandite growth occurred at similar depths and from waters of similar isotopic composition (618O = 1-6‰ SMOW) throughout most of the Southern Permian Basin. However, the timing of illite growth varied between areas and corresponded to periods to tectonic activity.

Paleomagnetic Orientation of Fractures and Bedding in Rotliegende and Zechstein Cores from the Southern Permian Basin, North Sea

Fractures and bedding in Rotliegende sandstone and Zechstein dolomite cores of the southern Permian Basin can be accurately oriented using our standard paleomagnetic core-orientation technique. In recent project involving vertical, deviated, and horizontal wells, we have paleomagnetically oriented 1874 ft of Rotliegende and Zechstein cores from 113 intervals ranging in length from 8 in. to 119 ft. Accuracy of our paleomagnetic core-orientations can be demonstrated by (1) consistency in fracture patterns derived from vertical and horizontal cores near the same reservoir location, (2) consistency in Rotliegende paleowind directions derived from paleomagnetically oriented cores with paleowind directions derived from mechanically oriented cores from the southern Permian Basin, and (3) agreement between structural dip determined from paleomagnetically oriented interdune bedding planes with structural dip determined from paleomagnetically oriented strike-0parallel, strike-perpendicular, and bedding-plane fractures. Paleomagnetic orientation of Rotliegende cores can be especially cost effect. Excellent core recovery and use of long core barrels in the Rotliegende means that [open quotes]continuous intervals[close quotes] (defined as the maximum lengths of core that can be reliably reconstructed by fitting adjacent core pieces) of more than 100 ft long can be achieved by following our recommended core-handling procedures. We statistically average the same number of paleomagnetic plugmore » samples regardless of the length of a [open quotes]continuous interval.[close quotes] The paleomagnetic signals in Rotliegende sandstone and Zechstein dolomite are sufficiently stable that fractures and bedding can be paleomagnetically oriented even in slabbed cores drilled decades ago.« less

Introduction and review

The aim of this session of the conference was to elucidate the nature of the Permo-Carboniferous and older plays in the producing areas of NW Europe and to suggest areas and play types where new discoveries could be made. The introductory paper by Gerard et al. describes the main producing areas: Southern Permian Basin, Timan-Pechora and the Baltic Basin. Source-rock distribution from Cambrian to Permian times is discussed in terms of eustatic and super-continent cycles, while the obvious relation of areas of production to areas without late tectonic deformation is described. For the Southern Permian Basin, the general hydrocarbon habitat and the development of the Southern North Sea is well illustrated by Hollywood and Whorlow's paper on multiphase structural development and the review by Bailey et al. of the plays of the Silver Pit Basin. The paper on the Caister Field by Ritchie and Pratsides illustrates the details of stacked structural plays with reservoirs of Triassic and Carboniferous age, while the papers by Turner et al. on the Ravernspurn North Field and by Burri et al. on the Rotliegend of NW Germany highlight the complexity of some traps.These two key papers on Rotliegend fields demonstrate the relationships between diagenesis, stratigraphy and structural history with the presence of good reservoir and the definition of subtle trap geometries. The approaches used by these authors point the way to future exploration evaluation as the Southern Permian Basin becomes increasingly mature. That the detailed stratigraphic and sedimentological frame-works necessary in the search for subtle . . .

Permian-Triassic Paleogeography and Stratigraphy of the West Netherlands Basin

During the Permian, the present West Netherlands basin (WNB) was situated at the southernmost margin of the southern Permian basin (SPB). The thickness of Rotilegende sandstones therefore is very much reduced in the WNB. The relatively thin deposits of the Fringe Zechstein in the WNB, however, also contrast strongly in sedimentary facies with thick evaporite/carbonate alternations in the main SPB to the north, although the classic cyclicity of Zechstein deposition still can be recognized. The Fringe Zechstein sediments are mainly siliciclastic and interfinger with both carbonates and anhydrites toward the evaporite basin. End members are thin clay layers that constitute potential seals to underlying Rotliegende reservoirs and relatively thick sandstones (over 100 m net sand) in the western part of the WNB. Nevertheless, favorable reservoir/seal configurations in the Fringe Zechstein seem to be sparse because only minor hydrocarbon occurrences have been proven in the area to date. The situation is dramatically different for the Triassic in the WNB. The [open quotes]Bunter[close quotes] gas play comprises thick Fringe Buntsandstein sandstones (up to 250 m), vertically sealed by carbonates and anhydritic clays of the Muschelkalk and Keuper formations. The Bunter sandstones are largely of the same age as the classic Volpriehausen, Detfurth,more » and Hardegsen alluvial sand/shale alternations recognized elsewhere, but the upper onlapping transgressive sands and silts correlate with evaporitic clays of the Roet basin to the north. A total volume of 65 x 10[sup 9]m[sup 3] of gas has so far been found in the Triassic Bunter sandstones of the WNB.« less

Sequences and Cyclicities in the Upper Rotliegende of the Netherlands

A regional study of released well and core data from the Netherlands on- and offshore territory, performed by InterGeos, has revealed the preservation of supersequences, sequences, and short-term cyclicities in upper Rotliegende deposits. These have always been interpreted as formed in an inland basin, disconnected from marine influence. Sequences and cyclicities are reflected in vertical sections by correlatable alternations of lithofacies, which can be observed in cores and wireline logs. The lithofacies shifts, which often show a marked periodicity, are interpreted as caused by climatic fluctuations. This implies that sequence stratigraphic methods can be used also in the southern Permian basin, even though direct marine influence is dubious. Supersequences, sequences, and cyclicities each have specific applications in Rotliegende exploration. Supersequences serve best for rough regional correlation and prediction of sand and clay. Sequences are suited for detailed regional or field correlation and studies of the detailed distribution of (un) favorable lithofacies, e.g., within a field. the correlation of sequences has revealed that existing lithostratigraphy is diachronous in character. Short-term cycles can be used to make estimates of net sedimentation rates and sequence durations, because the periods of these Milankovitch cycles are known. Short-term cyclicities are preserved better in the desertmore » red beds of the rotliegende than in coastal plain deposits such as the Carboniferous, where they are overprinted heavily by longer term cyclicities.« less

Process-Based Modeling of the Aeloian Environment at the Dune Scale

Process-based models are quantitative models that simulate the physical process of sedimentation with the objective of reconstructing the spatial distribution, stratification, and properties of the subsurface. In this study, a two-dimensional, process-based model of the aeolian environment, at the dune-interdune scale, has been developed. Sedimentation is governed by the variation of wind velocity over the topography, which is calculated analytically. Velocity calculations are coupled to a sediment transport equation, to determine where erosion and deposition occur. The resulting change in topography determines a new velocity field, which is then calculated. Features that the model simulates include ripple formation and dune migration, as well as the resulting internal sedimentary structures. Process-based models can be used as tool to help interpret structures in ancient formations. This model has been applied specifically to reconstruct dune-interdune sequences observed in cores from the Rotliegendes, localized in the southern Permian basin (North Sea). The interdune strata are characterized by a low permeability. A flow simulation has been done on the aeolian section generated by the model, showing the effect of these heterogeneities on fluid flow.

Relation Between Facies, Diagenesis, and Reservoir Quality of Rotliegende Reservoirs in North Germany

In north Germany, the majority of Rotliegende gas fields is confined to an approximately 50 km-wide east-west-orientated belt, which is situated on the gently north-dipping flank of the southern Permian basin. Approximately 400 billion m[sup 3] of natural gas has been found in Rotliegende reservoir sandstones with average porosities of depths ranging from 3500 to 5000 m. Rotliegende deposition was controlled by the Autunian paleo-relief, and arid climate and cyclic transgressions of the desert lake. In general, wadis and large dunefields occur in the hinterland, sebkhas with small isolate dunes and shorelines define the coastal area, and a desert lake occurs to the north. The sandstones deposited in large dunefields contain only minor amounts of illite, anhydrite, and calcite and form good reservoirs. In contrast, the small dunes formed in the sebkha areas were affected by fluctuations of the desert lake groundwaters, causing the infiltration of detrital clay and precipitation of gypsum and calcite. These cements were transformed to illite, anhydrite, and calcite-II during later diagenesis, leading to a significant reduction of the reservoir quality. The best reservoirs occur in the shoreline sandstones because porosity and permeability were preserved by early magnesium-chlorite diagenesis. Since facies controls diagenesis and consequently reservoirmore » quality, mapping of facies also indicates the distribution of reservoir and nonreservoir rocks. This information is used to identify play area and to interpret and calibrate three-dimensional seismic data.« less

Development of the Permian-Triassic Sequence in the Basin Fringe Area, Southern Netherlands

Geological studies in the fringe area of the southern Permian basin led to new insights in the distribution and development of the Permian-Triassic sequence. During the Permian, the fringe area formed a platform, attached to the London-Brabant Massif, while during the Triassic it is characterized by strongly subsiding half grabens. In the southern Netherlands, Rotliegende sandstones and conglomerates have a much wider distribution than previously recognized. The Rotliegende deposits are capped by claystones and carbonates of the Upper Permian Zechstein. In the offshore, an important feeder system of clastics from the London-Brabant Massif was active during deposition of the Rotliegende and the Zechstein. In course of time, the location of major sandstone deposition shifted westward. Deposition of the Triassic Buntsandstein was controlled by the development of a large feeder system, which transported clastics from the Vosges northward, through the Roer Valley Graben and West netherlands Basin into the Off Holland Low. This system was responsible for the deposition of the economically important sheet sandstones of the Volpriehausen, Detfurth, Hardegsen, and Solling formations. A regional unconformity occurs below the Solling Formation. The sandstones are capped by claystones, evaporites, and sandstones of the Rot Formation. During deposition of the Muschelkalk, the differencesmore » in subsidence decreased and shallow marine sediments are interbedded with evaporites. Several unconformities occur within the Keuper. In the previous half grabens in the southern Netherlands, the Keuper is incomplete, which may be indicative for a possible reversal of the tectonic movements during this period.« less

Interpreting the Upper Carboniferous of the Dutch Cleaver Bank High

The Dutch Cleaver Bank High is an area that has undergone a uniform structural and sedimentological history since the Lower Carboniferous. It is characterized by an absence of Upper Triassic and Jurassic sediments, and covers an area of some 8000 km 2 . The basement in this part of the Southern North Sea owes its origin to closure of the Tornquist Sea during the Acadian (late Caledonian) orogeny. NW–SE compressive structures were formed which were subsequently reactivated as normal faults during rifting in the late Devonian–Dinantian. Visean carbonates developed over most of the Dutch Cleaver Bank High but towards the Elbow Spit High, Yoredale facies sediments were deposited. After rifting ceased in the early Namurian, a thermal sag basin evolved which was infilled by marine and deltaic sediments of the Millstone Grit sequence. The Coal Measures sequence of Westphalian to early Stephanian age unconformably or disconformably overlies the Millstone Grit. The Coal Measures sequence consists of fluviatile and associated sediments and is at least 1200 m thick. It contains several regionally correlatable lithological units of remarkably constant thickness. Sedimentation appears to have been uninterrupted during the entire Westphalian in this area. This implies that subsidence was sufficiently rapid to counterbalance the tendency for erosion on the continental plain during periods of sea-level fall. The high rate of subsidence is thought to be due to flexure in the foreland of the Variscan mountain belt. The axis of the basin lies to the south of the Dutch Cleaver Bank High. The source area for the sediment was to the north. From base to top of the Coal Measures sequence there is a change in lithology from lacustro-deltaic to poorly drained continental sediments to well-drained continental sediments, indicative of overall terrestrial progradation. However, within this overall trend, higher frequency fluctuations in base level have had significant impact on lithological variations on a scale of 25–375 m. During periods of rapid base-level rise, thick packages of fluviatile sediment were deposited due to a high rate of accommodation on the continental plain. These fluviatile packages are equivalent to transgressive systems tracts (TST). Mapping reservoir intervals within the Coal Measures sequence is made possible by the uniformity of the strata over the entire Dutch Cleaver Bank High. The most important correlative unit is a TST of Westphalian A/B age which consists of a constant 375 m of poorly drained alluvial plain sediment. It represents a fluvial complex with dimensions greater than the Dutch Cleaver Bank High. The unit is highly reflective on seismic due to constructive interference of the seismic wavelet within fining-upwards parasequences of approximately 25 m thickness. A major rifting event occurred at the end of the Stephanian leading to extension, uplift and igneous activity. In places on the Dutch Cleaver Bank High more than 1000 m of the Coal Measures sequence were removed by erosion. The Elbow Spit High was formed at this time as a major fault-rotated tilt block, up-dip of the Dutch Cleaver Bank High. The Southern Permian Basin developed due to thermal sagging after rifting ceased. Three further inversion events have affected the area since the Permian.

Ranking of Geological Factors Controlling the Well Productivity in Rotliegende Gas Fields of the Dutch Offshore Area

Deeply buried Rotliegende sandstones deposited in the desert environment of the Southern Permian Basin form a primary target for hydrocarbon exploration and production in northwest Europe. In the Dutch offshore sector, some highly profitable Rotliegende fields are known. However, the success rate for exploration and field development is adversely affected by the variable Rotliegende reservoir quality, with tight gas sands and productive reservoirs units adjoining each other within fields. To improve the accuracy of reservoir performance forecasts, several case histories were analyzed and the observed well productivities correlated with key mineralogical and sedimentological parameters. The overriding influence of facies is evident: the textural characteristics of the Rotliegende sand determined not only the initial porosity after deposition, but influenced also the response of the rock to burial diagenesis. The mature, better sorted sandstone facies suffered least from diagenesis, whereas the immature lithologies lost much of their porosity and permeability by pervasive diagenetic cementation and clay mineral growth. The relative influence of burial/temperature history and mineralogical and sedimentological factors on well productivity was evaluated with multivariate statistics. Three field studies from the Dutch offshore area demonstrated that the well productivity can be modeled as a function of a limited number of geologicalmore » factors. Testing the statistical formula with additional wells showed that the multivariate method improves the accuracy of reservoir quality predictions.« less

Pyrolysis and Hydrocarbon Source Bed Potential of the Upper Devonian Woodford Shale, Hovey Channel, Southern Permian Basin, West Texas

The Upper Devonian Woodford Shale in the Hovey Channel area, southern Permian basin, is 50 m thick and composed largely of brown to black, pyritic, spore-bearing, organic-rich, fissile shale an chert. Total organic carbon, distillable hydrocarbons, genetic potential, organic carbon index, hydrogen index, temperature of maximum hydrocarbon generation, and kerogen transformation index of the Woodford Shale suggest a matured to overmatured, gas-generating source bed. The total organic carbon content of the formation ranged from a low of 0.77% in the cherty samples to a high of 4.59% in a shaley sample, averaging 2.18%. Distillable hydrocarbon content of the samples is fairly high (averaging 1.72 mg HC/gm{degree} rock), varying from 0.90 mg HC/gm{degree} rock to 3.22 mg HC/gm{degree} rock. Genetic potential evaluated in terms of both residual and total generative potential showed above average potential, averaging 3.25 mg HC/gm{degree} rock for the residual and 4.90 mg HC/gm{degree} rock for the total, respectively. Live organic carbon index values ranged from 11-28%, characterizing the formation as a moderate to good source bed. Hydrogen index values ranged from 73 mg HC/gm{degree} C org to 155 mg HC/gm{degree} C org, suggesting overmaturity and gas-generation potential of the source bed. Temperature of maximum hydrocarbon generation valuesmore » and kerogen transformation ratio values (averaging 0.34) also indicate overmatured nature of the Woodford Shale.« less

Hydrocarbon Habitat and Future Exploration in Eastern Europe

Eastern Europe can be subdivided into several tectonic units. All of these are productive. The Pannonian-Carpathian system is controlled by a roll back of a subducted plate resulting in the formation of the Carpathian fold belt and the Pannonian basin. The topography of the Carpathian fold belt was less than an Alpine-type fold belt, and is associated with a thicker flysch-dominated foredeep. Hydrocarbons occur in all of these tectonic settings. The Southern Permian basin which extends eastward from the southern North Sea into Poland is well known for its giant gas reserves in Rotliegende and Zechstein reservoirs. Overlying the Permian basin are several individual oil and gas producing Mesozoic basins which were inverted during the Late Cretaceous. Their Eastern European equivalent is the Polish trough along the Tornquist line. The future exploration of Eastern Europe primarily is in underexplored production plays that require application of new technologies and new capital investments. Additionally, new hydrocarbon plays, yet to be realized, await discovery. The search for new hydrocarbon plays should be initiated by regional studies, and by asking new questions and applying new ideas and concepts. Lately, for example, this has been done in the Pannonian-Carpathian region, resulting in a new tectonicmore » understanding and a much better interpretation of the various hydrocarbon habitats. Similar approaches should be undertaken to evaluate the hydrocarbon potential of other Eastern European basins. The future of exploration in Eastern Europe is in understanding its complex, compressive tectonic environments.« less

Zechstein reservoirs in The Netherlands

Abstract Thick, shallow-water gypsum banks, formed during Zechstein times at the site of regional palaeohighs along the margins of the Southern Permian Basin, acted as foundations for shallow-water carbonate platforms. The carbonate platform margins, in particular their northern to northeastern windward margins, were characterized by deposition of grainstones dominated by aragonitic ooids, pisoids, intraclasts and bioclasts, while muddy, stromatolitic carbonates and nodular anhydrites accumulated on tidal flats and in lagoons behind the platform margins. Usually the platforms are surrounded by thick slope deposits consisting of carbonate mudstone and redeposited intervals, but locally the platforms deepened gradually and changed, without intervening slope deposits, into a shallow-marine facies devoid of ooids. The slope aprons grade basinwards into thin successions of finely laminated carbonate. Porosity creation in Zechstein carbonates was due to meteoric-water leaching and to subsurface leaching by carbon dioxyde-enriched formation waters during burial. Porosity destruction was mainly caused by anhydrite pore plugging and subsurface calcitization. The best reservoir properties are found in the unrestricted platform facies and in parts of the slope facies. Fractures are important in Zechstein carbonates; their presence is a prerequisite for high production rates in gasfield wells.

Development of N.W. Europe’s Southern Permian Gas Basin

Summary The Southern Permian Basin contains over 4.1 × 10 12 m 3 of recoverable non-associated gas, most of which is found in Early Permian Rotliegend sandstones. The most important source of this gas is Carboniferous coal deposited in a proto-Tethys foredeep basin. Where the coals were overlain by Rotliegend desert sands and by Late Permian Zechstein halite, the scene was set for creating important reservoirs for gas. Deformation of the reservoir and seal to create a trap, and the generation and migration of Carboniferous gas, resulted from the structural history of the area, which was shaped in part by underlying crustal blocks and zones of crustal weakness. Much of this history was related to events most clearly expressed beyond the limits of N.W. Europe in the North Atlantic and Tethys: (1) E.-W. tension, which gave rise to the oblique-slip-induced subsidence of many sub-basins within the Southern Permian Basin, caused the consecutive creation of the Viking-Central graben system and Rockall Trough, and ended with the crustal spreading of the Atlantic Ocean; and (2) the early Mesozoic opening of Tethys, its Cretaceous closure and ensuing Alpine orogeny, which induced inversion in many areas of the Southern Permian Basin that are now gas bearing.

Basin-Edge Diapirism and Updip Salt Flow in Zechstein of Southern North Sea

Some unusual and interesting structural geometries have been recognized on seismic lines recorded in the United Kingdom sector of the southern Permian basin of the North Sea. They seem to be the result of diapirism at the northern edge of the (Upper Permian) Zechstein salt basin, and involve the preservation of a 75 km (47 mi) long prism of a younger Mesozoic sequence replaced elsewhere by the widespread Jurassic/Cretaceous late Kimmerian unconformity. It is suggested that the diapiric features described are due to the movement of salt toward the basin edge having been dammed by a change in facies from basinal halite to shelf lithologies over a short distance.

Late Paleozoic Structural Evolution of Permian Basin: ABSTRACT

The Permian basin of West Texas and New Mexico is one of the premier hydrocarbon provinces of the world; nonetheless, little regional subsurface structural information about it has been published. Mapping at 1:250,000 on the Ellenburger horizon (Lower Ordovician), compiled for the Tectonic Map of Texas, discloses the overall geometry of Paleozoic deformation in the area. The southern Permian basin is underlain by the NNW-trending Central Basin disturbed belt of Wolfcamp age (Lower Permian), the deep Delaware basin to its west, and the shallower Midland basin to its east. The disturbed belt is highly segmented with zones of left-lateral offset. Major segments from south to north are: the Puckett-Grey Ranch zone; the Fort Stockton uplift; the Monahans transverse zone; the Andector ridges and the Eunice ridge; the Hobbs transverse zone; and the Tatum ridges, which abut the broad Roosevelt uplift to the north. East-west compression is inferred, with shortening increasing from the Tatum ridges south to the Fort Stockton uplift. The segment boundaries and transverse elements are inferred zones of strike-slip faulting. These fault zones extend both southeast and west of the disturbed belt into discrete strike-slip faults with local uplifts in compressive bends (such as the Big Lake uplift). The Midland basin is much shallower than the Delaware basin, and the uplift-to-basin transition is gradual. A belt of subtle domes and anticlines, extending northeast from Andrews County, overlies a major basement discontinuity (the Grenville Front). The disturbed belt may have originated along rift zones of either Precambrian or Cambrian age. The extent of Lower and Middle Pennsylvanian deformation is unclear; much of the Val Verde basin-Ozona arch structure may have formed then. The main Wolfcamp deformation overthrust End_Page 474------------------------------ the West Texas crustal block against the Delaware block, with local denudation of the uplifted edge and eastward-directed backthrusting into the Midland basin. Later in the Permian, the area was the center of a subcontinental bowl of subsidence--the Permian basin proper. The disturbed belt formed a pedestal for the carbonate accumulations which created the Central Basin platform. The major pre-Permian reservoirs of the Permian basin lie in large structural and unconformity-bounded traps on uplift ridges and domes. Further work on the regional structural style may help to predict fracture trends, to assess the timing of oil migration, and to evaluate intrareservoir variations in the overlying Permian giant oil fields. End_of_Article - Last_Page 475------------

Early Permian (Rotliegendes) palaeowinds of the north sea

The classical desert sediments of the Early Permian Upper Rotliegendes were deposited in a post-Variscan basin that extends from eastern England to the Russo-Polish border, which has been referred to as the Southern Permian Basin. These sediments comprise four main depositional facies: fluvial (wadi), aeolian, sabkha and lacustrine. The lacustrine facies includes important bedded halites. The Southern Permian Basin is separated from the smaller Northern Permian Basin by the fragmented Mid North Sea-Ringkøbing-Fyn system of highs: and beyond the Grampian Spur lies the even smaller Moray Firth Basin. These latter basins contain rock sequences similar to those of the southern basin, with the exception that bedded halite has not been recognised in the lacustrine facies. The Rotliegendes of the southern basin apparently conforms to deposition in a Northern Hemisphere tradewind desert similar to the modern southern Sahara. Apart from regional palaeomagnetic considerations and the occurrence of sediments of a non-arid nature south of the Variscan Highlands, the strongest evidence supporting this interpretation is the pattern of palaeowind directions deduced from the orientations of dune bedding. This is seen in both outcrop and core and derived from the continuous dip-meter logs of many North Sea wells by plotting the poles to the bedding attitudes on polar nets. Palaeowind directions have also been deduced from the dip-meter logs of wells drilled in the Northern and Moray Firth Basins. In general, they indicate winds that blew in a direction opposed to those of the Southern Basin. Thus an area of high barometric pressure seems to have been located over the Mid North Sea High. When the bedding attitudes in a well are roughly unidirectional, an origin on a transverse dune is normally indicated. With some wells, the indicated palaeowind direction is regionally anomalous. In many cases this can be rectified if the well is presumed to have drilled down through one flank of a seif dune. The indicated palaeowind is thus changed by up to 90° and then commonly fits the regional pattern. These early Permian desert basins were smaller and closer to the equator than the modern Sahara. Their location in the centre of a continent between the Caledonian and Variscan mountain ranges suggests the analogy of a more tropical equivalent of the Central Asian Takla Makan Desert.

Depositional facies, diagenetic clay minerals and reservoir quality of Rotliegend sediments in the Southern Permian Basin (North Sea): a review

AbstractThe Southern Permian Basin of the North Sea represents an elongate E-W oriented depo-centre along the northern margin of the Variscan Mountains. During Rotliegend times, three roughly parallel facies belts of a Permian desert developed, these following the outline of the Variscan Mountains. These belts were, from south to north, the wadi facies, the dune and interdune facies, and the sabkha and desert lake facies. The bulk of the gas reservoirs of the Rotliegend occur in the aeolian dune sands. Their recognition, and the study of their geometry, is therefore important in hydrocarbon exploration. Equally important is the understanding of diagenesis, particularly of the diageneticaily-formed clay minerals, because they have an important influence on the reservoir quality of these sands. Clay minerals were introduced to the aeolian sands during or shortly after their deposition in the form of air-borne dust, which later formed thin clay films around the grains. During burial diagenesis, these clay films may have acted as crystallization nuclei for new clay minerals or for the transformation of existing ones. Depending on their crystallographic habit, the clay minerals can seriously affect the effective porosity and permeability of the sands.

Geologic Evolution of North Sea and Its Tectonic Framework

The tectonic history of the North Sea area can be subdivided into five stages: 1. Caledonian geosynclinal stage (Cambrian-Devonian). Metamorphic and intrusive rocks of Caledonian age form the basement complex for much of the North Sea area. The northeastern boundary of the Caledonian foldbelt at this stage cannot be defined more closely than as an apparent trend from the central North Sea through northern Germany into Poland. 2. Variscan geosynclinal stage. Devonian and Carboniferous deposition transgressed from the south over the eroded Caledonides and reached maximum thickness in the southern North Sea, an area which formed part of the Variscan foredeep. 3. Permian-Triassic intracratonic stage. Following the Variscan orogeny large parts of the North Sea were occupied by the rapidly subsiding intracratonic Northern and Southern Permian basins. These basins contain a thick sequence of clastic and evaporite deposits. 4. Rifting, taphrogenic stage. Development of the North Sea rift system started during the Triassic and dominated the paleogeographic setting of the area during the Jurassic and Cretaceous. The evolution of the North Sea rift is related to the development of the Arctic North Atlantic rift zone. The latter reached the stage of crustal separation in the early Tertiary, at which time the North Sea rift became inactive. 5. Tertiary, postrifting stage of regional basin subsidence. With the termination of rifting movements in the North Sea the area became subject to regional subsidence leading to the development of a symmetrical, saucer-shaped intracratonic basin. The late Tertiary Rhone-Rhine rift system does not extend into the North Sea and postdates the North Sea rift.

Late Paleozoic Structural Directions in Southern Permian Basin, West Texas and Southeastern New Mexico

This study shows the possibility of extensive lateral movement along pre-Permian faults in the Permian basin and determines, largely from subsurface evidence, the direction of the forces responsible for Pennsylvanian and Permian tectonic deformation. A review of the relation of faulting to orthogonal stress axes shows that changes in the geometry and genesis of faults are the consequences of changes in the orientation of the stress axes. Two tectonic systems can be discerned. One consists of folds and faults. The fold axes strike N35°W, whereas the fault systems include one that strikes N55-80°E with indications of right lateral movement and another that strikes N50-65°W with left lateral movement. Thus the fault pattern indicates stress in a nearly east-west direction and the folds indicate direction of stress to be about N65°E. The age of this deformation is thought to be early Late Mississippian to late Middle Pennsylvanian. Another fault system which seems to be younger is composed of faults striking slightly west of north. The most prominent example of this system is the West Platform fault, the movement on which is right lateral with a displacement of tens of miles. The stress giving rise to this system seems to have been nearly north and south and may be related to the force that powered the early and middle Wolfcampian (Permian) thrusting on the south in the Marathon salient of the Ouachita belt. The jagged pattern of the eastern edge of the Central Basin platform may be caused by conjugate faults of this system. The later tectonic history of the region is marked by relaxation of stress and subsequent normal fault movement on the previously formed fault planes. This movement deepened preexisting basins; reefs and other sedimentary features formed around the peripheries of these depressions. Formation of salt pans in later Permian time was related to minor movement along the faults. Remnants of the basins persisted throughout Triassic time, but were obliterated by the great change in geography during Cretaceous time. Renewed movement along the old lines of weakness during Tertiary time caused the eastward tilting of the Delaware basin and the formation of grabens along its west edge. Deep channels in Pleistocene and Holocene sand deposits in the eastern part of the Delaware basin reflect soluti n of the underlying Permian evaporites over these lines.